March 18, 2008

Divergence of modern humans and Neanderthals

The time of the split between Neanderthals and modern humans has been previously calculated by comparing modern human and Neanderthal DNA. In this paper, the authors use cranial morphology to arrive at an independent estimate of the divergence time between the two species. Modern human populations have differentiated cranially as they expanded from Africa. By comparing this differentiation with that observed between modern humans and Neanderthals, the authors were able to arrive at an estimate of the divergence between the two species.

New research led by UC Davis anthropologist Tim Weaver adds to the evidence that chance, rather than natural selection, best explains why the skulls of modern humans and ancient Neanderthals evolved differently. The findings may alter how anthropologists think about human evolution.

Weaver's study appears in the March 17 issue of the Proceedings of the National Academy of Sciences. It builds on findings from a study he and his colleagues published last year in the Journal of Human Evolution, in which the team compared cranial measurements of 2,524 modern human skulls and 20 Neanderthal specimens. The researchers concluded that random genetic change, or genetic drift, most likely account for the cranial differences.

Erik Trinkaus is a Neandertal expert at Washington University in St. Louis.

The new study by Weaver's team "is valid in indicating that those aspects of the human cranium that are likely to be governed by random processes, such as drift, are in agreement with … genetic analyses," he said.

"Both of them are [also] in general agreement with the fossil record, which indicates that you start getting divergent aspects of human anatomy in Africa and Europe 300,000 to 500,000 years ago."

This finding may contradict a common belief that humans won out over Neanderthals because they acquired helpful physical changes in their skulls.

...

Erik Trinkaus, an anthropologist at Washington University in St. Louis who did not work on the study, said the research cannot establish absolutely that natural selection did not create skull differences between Neanderthals and modern humans, or even that the divergence date of 370,000 years is accurate, because both dating methods are only looking at chance mutations.

Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0709079105

Close correspondence between quantitative- and molecular-genetic divergence times for Neandertals and modern humans

Timothy D. Weaver et al.

Recent research has shown that genetic drift may have produced many cranial differences between Neandertals and modern humans. If this is the case, then it should be possible to estimate population genetic parameters from Neandertal and modern human cranial measurements in a manner analogous to how estimates are made from DNA sequences. Building on previous work in evolutionary quantitative genetics and on microsatellites, we present a divergence time estimator for neutrally evolving morphological measurements. We then apply this estimator to 37 standard cranial measurements collected on 2,524 modern humans from 30 globally distributed populations and 20 Neandertal specimens. We calculate that the lineages leading to Neandertals and modern humans split {approx}311,000 (95% C.I.: 182,000 to 466,000) or 435,000 (95% C.I.: 308,000 to 592,000) years ago, depending on assumptions about changes in within-population variation. These dates are quite similar to those recently derived from ancient Neandertal and extant human DNA sequences. Close correspondence between cranial and DNA-sequence results implies that both datasets largely, although not necessarily exclusively, reflect neutral divergence, causing them to track population history or phylogeny rather than the action of diversifying natural selection. The cranial dataset covers only aspects of cranial anatomy that can be readily quantified with standard osteometric tools, so future research will be needed to determine whether these results are representative. Nonetheless, for the measurements we consider here, we find no conflict between molecules and morphology.

6 comments:

Trikhaus was the same professor claiming that some Cromagnon skull from Romania I think it was, was Neanderthal or some evidence of a hybrid.

Each new study, they either contradict eachother, or their opinions dont agree with their own research...even remembering C. Loring Brace comments that time.Oddballs sometimes they are imo.

Also isnt the Mladec 1 from the Czech Republic, dated to 31,000BP,which is the earliest first known Caucasoid skull? and the only known Mongoloid skull is something like 7,000 years old, while the Negroid is around 12-15,000 years old. So I wonder where they get all these tensor hundreds of thousands of years from?

The first Mongoloid skulls lie deep in the soil of Siberian taiga. They still wait for their discovery. It was only due to the Neolithic population expansion of the Chinese (from North-Eastern China) that Mongoloid phenotype spreaded all over East and South-East Asia. Paleolithic Chinese looked more like Ainu or Polynesians, not like Mongoloids.

When I look at the morphological differences between neanderthal and modern humans I am always struck by the fact that the two look as if they are the same species but differentiated by the degree of exposure to testoterone or some other growth factor as much as anything else; as the difference between the skull of a wolf and a golden retriever. Were I capable of conducting a breeding experiment with a population of primates in order to select for "socialability" I wonder if I wouldn't see morphological differences as was observed in the well known breeding program with captive foxes in Siberia. It was, I seem to recall, attributed to pressures similar to neotony in which late developing aggressive behaviors came along with a suite of other traits. Would Chimps selected for their apparent retention of youthful playfullness become more like Bonobos? More gracile, and perhaps the genetic potentials might be more directed towards increased brain size as one trait of the young is the sustained growth of the brain.

I think this is, in some ways, highly speculative. The study itself doesn't really say anything new; it just confirms what is known about the divergence times between the two human populations. And I would also agree with the comments made by one other poster, that a lot of these studies seem to contradict one another. But then, if you take someone like Trinkaus, he has certain views about where Neandertals fit in the human evolutionary scheme of things, and the people who did this study, seem to have other views. These views tend to influence their conclusions, which is one reason why these studies seem endless and contradictory.Anne G

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